Whittled-down woods

Plant communities and invasive species in forest fragments

When continuous forests are carved into smaller fragments, plant communities in the remnants are affected by the ensuing environmental changes. It also influences the ecology of native and alien species differently. Understanding which species persist and which ones decline and disappear is crucial for conservation.

Plants in fragments and continuous rainforest

How does fragmentation affect plants? To answer this, we studied plant communities in forest fragments and continuous forests in the Anamalai Tiger Reserve. By systematic study of trees, lianas (woody climbers), shrubs, and herbs, in sample plots, we found interesting trends. Rainforest trees tended to decrease in diversity and density in smaller and more disturbed fragments, while lianas increased in moderately disturbed sites. Shrubs and herbs in the forest understorey showed a tendency to increase in density and diversity in smaller fragments overall, but this was partly because decline in native evergreen species was masked by an increase in weedy, alien species, that appeared to thrive in disturbed sites. Still, even in the relatively small sample area (4 ha for trees and lianas, 0.16 ha for understorey plants), the rainforests retained an astonishing plant diversity. There were 144 tree species, 60 liana species, and 108 understorey plant species; of these 70 species are endemic to the Western Ghats.

Invasion by coffee and other alien species

Where forest fragments abut farms and plantations, there is a possibility that crop plants may run wild into forests. While this does not happen with many cultivated species, plants such as coffee which has its origins in African forests and which can grow under the shade of trees, may manage to invade forests. In the Anamalai hills, we studied invasion by coffee and other alien species from the edge of forest fragments abutting coffee plantations into the interior. Robusta coffee was found to be highly invasive in both relatively undisturbed and disturbed forests. Arabica coffee, in comparison, was less invasive, and occurred more in disturbed fragments. Other invasive alien species such as Lantana camara, Chromolaena odorata, and Maesopsis eminii, showed divergent patterns related to disturbance history and present status.

The effects of fragmentation and overstorey tree diversity on tree regeneration were assessed in tropical rain forests of the Western Ghats, India. Ninety plots were sampled for saplings (1–5 cm diameter at breast height (dbh); 5×5-m plots) and overstorey trees (>9.55 cm dbh; 20×20-m plots) within two fragments (32 ha and 18 ha) and two continuous forests. We tested the hypotheses that fragmentation and expected seed-dispersal declines (1) reduce sapling densities and species richness of all species and old-growth species, and increase recruitment of early-successional species, (2) reduce the prevalence of dispersed recruits and (3) increase influence of local overstorey on sapling densities and richness. Continuous forests and fragments had similar sapling densities and species richness overall, but density and richness of old-growth species declined by 62% and 48%, respectively, in fragments. Fragments had 39% lower densities and 24% lower richness of immigrant saplings (presumed dispersed into sites as conspecific adults were absent nearby), and immigrant densities of old-growth bird-dispersed species declined by 79%. Sapling species richness (overall and old-growth) increased with overstorey species richness in fragments, but was unrelated to overstorey richness in continuous forests. Our results show that while forest fragments retain significant sapling diversity, losses of immigrant recruits and increased overstorey influence strengthen barriers to natural regeneration of old-growth tropical rain forests.

Journal Article

2015

Invasive alien species in relation to edges and forest structure in tropical rainforest fragments of the Western Ghats

The impact of invasive alien species on native ecosystems is a major conservation issue in the tropics. This study in the rainforest fragments of Anamalai hills, in the Western Ghats biodiversity hotspot, assessed the effects of distance from edges and forest structure on the occurrence and abundance of three invasive alien species (Chromolaena odorata, Lantana camara, and Maesopsis eminii). Replicate line transects were laid from the edges into the interiors of four fragments varying in disturbance level and area (32 ha – 200 ha). Densities of alien species in the protected site were lower than in the three disturbed fragments. Maesopsis eminii occurred at highest density (382 trees/ha) in the highly disturbed site where it also showed prolific regeneration (1510 saplings/ha). The invasive alien species showed no clear edge-to-interior pattern, instead their abundance appeared to be localized and related in a site-specific manner to disturbances such as presence of Eucalyptus plantation, canopy openings, and trails.

While the conservation impacts of invasive plant species on tropical biodiversity is widely recognised, little is known of the potential for cultivated crops turning invasive in tropical forest regions. In the Western Ghats biodiversity hotspot, India, fragmented rainforests often adjoin coffee plantations. This study in the Anamalai hills assessed the effects of distance from edges and forest structure on the occurrence and abundance of shade-tolerant coffee (Arabica Coffea arabica and Robusta C. canephora) in four fragments (32–200 ha) using replicate line transects laid from the edges into the interiors. The coffee species cultivated in adjoining plantations was more abundant than the other coffee species inside study fragments, showing a clear decline in stem density from edge (0 m) to interior (250 m), suggesting the influence of propagule pressure of adjoining plantations, coupled with edge effects and seed dispersal by animals. Significant positive correlations of coffee density with canopy cover indicate the potential threat of coffee invasion even in closed canopy rainforests. Stem density of Coffea arabica (150–1,825 stems/ha) was higher in more disturbed fragments, whereas Coffea canephora had spread in disturbed and undisturbed sites achieving much higher densities (6.3–11,486 stems/ha). In addition, a negative relationship between C. canephora and native shrub density indicates its potential detrimental effects on native plants

Changes in tree, liana, and understory plant diversity and community composition in five tropical rain forest fragments varying in area (18–2600 ha) and disturbance levels were studied on the Valparai plateau, Western Ghats. Systematic sampling using small quadrats (totaling 4 ha for trees and lianas, 0.16 ha for understory plants) enumerated 312 species in 103 families: 1968 trees (144 species), 2250 lianas (60 species), and 6123 understory plants (108 species). Tree species density, stem density, and basal area were higher in the three larger (> 100 ha) rain forest fragments but were negatively correlated with disturbance scores rather than area per se. Liana species density, stem density, and basal area were higher in moderately disturbed and lower in heavily disturbed fragments than in the three larger fragments. Understory species density was highest in the highly disturbed 18-ha fragment, due to weedy invasive species occurring with rain forest plants. Nonmetric multidimensional scaling and Mantel tests revealed significant and similar patterns of floristic variation suggesting similar effects of disturbance on community compositional change for the three life-forms. The five fragments encompassed substantial plant diversity in the regional landscape, harbored at least 70 endemic species (3.21% of the endemic flora of the Western Ghats–Sri Lanka biodiversity hotspot), and supported many endemic and threatened animals. The study indicates the significant conservation value of rain forest fragments in the Western Ghats, signals the need to protect them from further disturbances, and provides useful benchmarks for restoration and monitoring efforts.

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